Organic light-emitting diodes (OLED's) are an exciting technology that may find uses a number of products. However, these devices still have a number of hurdles to overcome to attain widespread use.
Extensive effort has been focused on the development of high efficiency devices. The introduction of heterojunctions has been shown to greatly improve the luminous efficiency of OLED's by C. W. Tang and S. A. VanSlyke (Appl. Phys. Lett. 51, 913 (1987)). Heterojunction OLED's are often composed by multilayers of amorphous organic thin films, sandwiched between two electrodes. Conventionally, the device structure has consisted of a hole injecting layer (HIL), a hole transporting layer (HTL), a light emitting layer (EML), an electron transporting layer (ETL), and an electron injecting layer, as well as an ITO, or other substantially transparent conductor, anode and a cathode (i.e. an ITO/HIL/HTL/EML/ETL/EIL/cathode structure). Usually, the HTL possesses a highest occupied molecular orbital (HOMO) and a lowest unoccupied molecular orbital (LUMO) that are high compared to those of the ETL. Consequently, holes may be injected from the anode into the HOMO of the HTL, while electrons may be injected from the cathode into the LUMO of the ETL. In this heterogeneous OLED structure, built-in barriers are established for injected carriers due to the difference between the HOMOETL and the HOMOHTL and the difference between the LUMOHTL and the LUMOETL for holes and electrons, respectively.
Because the EML is sandwiched between the HTL and the ETL, these built-in barriers can confine electrons and holes in the EML. To increase the luminous efficiency, the EML may be doped with highly fluorescent organic dye as described by C. W. Tang, S. A. VanSlyke, and C. H. Chen in ELECTROLUMINESCENCE OF DOPED ORGANIC THIN FILMS (J. Appl. Phys. 65, 3610 (1989)).
Inserting an extra layer such as a hole/electron blocking layer may also enhance the effective built-in barrier of this OLED structure to confine, in the EML, holes/electrons for recombination as described by Y. Kijima, N. Asai, and S. Tamura in A BLUE ORGANIC LIGHT EMITTING DIODE (Jpn. J. Appl. Phys. 38, 5274 (1999)) and J. A. Hagen, W. Li, A. J. Steckl, and J. G. Grote in ENHANCED EMISSION EFFICIENCY IN ORGANIC LIGHT-EMITTING DIODES USING DEOXYRIBONUCLEIC ACID COMPLEX AS AN ELECTRON BLOCKING LAYER (Appl. Phys. Lett. 88, 171109 (2006)). However, heterojunction interfaces are known to incur device instability due to existence of high local electric fields established along the heterojunction interface. This potential issue is discussed in articles by V.-E. Choong, S. Shi, J. Curless, C.-L. Shieh, H.-C. Lee, J. Shen, and J. Yang in ORGANIC LIGHT EMITTING DIODES WITH A BIPOLAR TRANSPORT LAYER (Appl. Phys. Lett. 75, 172 (1999)) and V.-E. Choong, S. Shi, J. Curless, and F. So in BIPOLAR TRANSPORT ORGANIC LIGHT EMITTING DIODES WITH ENHANCED RELIABILITY BY LiF DOPING (Appl. Phys. Lett. 76, 958 (2000)). Excess carriers accumulating near the boundaries of the EML may thus lead to intrinsic degradation of organic materials as described by H. Aziz, Z. D. Popovic, N. X. Hu, A. M. Hor, and G. Xu in DEGRADATION MECHANISM OF SMALL MOLECULE-BASED ORGANIC LIGHT-EMITTING DEVICES (Science 283, 1900 (1999)) and Z. D. Popovic, H. Aziz, N. X. Hu, A. Ioannidis, and P. N. M. dos Anjos in SIMULTANEOUS ELECTROLUMINESCENCE AND PHOTOLUMINESCENCE AGING STUDIES OF TRIS(8-HYDROXYQUINOLINE) ALUMINUM-BASED ORGANIC LIGHT-EMITTING DEVICES (J. Appl. Phys. 89, 4673 (2001)).
One approach to eliminate this excess charge accumulation incorporates charge transporting materials into the EML to form an EML-HTL-ETL co-host emitter (CHE) instead of a single EML. As a result, the proposed OLED device structure becomes: ITO/HIL/HTL/EML:HTL:ETL/ETL/EIL/cathode. This structure has been described by a number of authors including: H. Aziz, et al. (Science 283, 1900 (1999)); Z. D. Popovic, et al. (J. Appl. Phys. 89, 4673 (2001); A. B. Chwang, R. C. Kwong, and J. Brown in GRADED MIXED-LAYER ORGANIC LIGHT-EMITTING DEVICES (Appl. Phys. Lett. 80, 725 (2002); T.-H. Liu, C.-Y. Iou, and C. H. Chen in DOPED RED ORGANIC ELECTROLUMINESCENT DEVICES BASED ON A COHOST EMITTER SYSTEM (Appl. Phys. Lett. 83, 5241 (2003)); S. W. Liu, C. A. Huang, J. H. Lee, K. H. Yang, C. C. Chen, and Y. Chang in BLUE MIXED HOST ORGANIC LIGHT EMITTING DEVICES (Thin Solid Films 453, 312 (2004)); and H. Kanno, Y. Hamada, and H. Takahashi in DEVELOPMENT OF OLED WITH HIGH STABILITY AND LUMINANCE EFFICIENCY BY CO-DOPING METHODS FOR FULL COLOR DISPLAYS (IEEE J. Select. Topics Quantum. Elect. 10, 30 (2004)). In this CHE system, a more complicated doping scheme (two hosts with one dopant) and a more complicated fabrication process, as compared to an EML only device, are typically used for color tuning.
This layer structure, however, leads to a more complicated doping scheme (two hosts with one dopant), a more complicated fabrication process, and as well as potentially low device stability. Thus, there has been demand for an exemplary OLED device structure that may be manufactured using a simplified process. The present invention is targeted to meet this demand with a novel, yet simple, EL device structure that may be used with OLED's and other EL semiconductor devices including inorganic EL devices.